1. Field of the Invention
The present invention relates to a signal processing device and method, and a program, and particularly relates to a signal processing device and method, and a program whereby detection can be carried out simpler in a sure manner.
2. Description of the Related Art
With digital modulation methods for transporting a digital signal with a predetermined allocated band, a signal obtained by modulating the amplitude or phase of a predetermined carrier according to data is transmitted. Of these digital modulation methods, in particular, a method for modulating amplitude is called “amplitude modulation”, and a method for modulating a phase is called “phase modulation”.
The amplitude modulation method, i.e., ASK (Amplitude Shift Keying) method, is a digital modulation method for modulating the amplitude of a carrier so as to correspond to data to be transmitted. With the amplitude modulation method, 1-bit data is made to correspond to the magnitude of amplitude, and among the amplitude modulation methods, there is known an OSK (On-Off Shift Keying) method for corresponding data to the presence/absence of amplitude, and so forth.
Also, the phase modulation method, i.e., PSK (Phase Shift Keying) method, is a digital modulation method for modulating the phase of a carrier so as to correspond to data to be transmitted. As for the phase modulation method, there are known the BPSK (Binary Phase Shift Keying) method for corresponding 1-bit data to a phase, QPSK (Quadrature Phase Shift Keying) method, and so forth.
For example, with BPSK, the phase of a carrier is made to correspond to 0 or n according to the logic value of baseband data to be transmitted. Specifically, if we say that the frequency of a carrier is taken as ω, and predetermined point-in-time is taken as t, the carrier can be represented with sin(ωt). If we say that the logic value 0 or 1 of baseband data to be transmitted is made to correspond to the phase 0 or n of a carrier, a BPSK signal in a section where the logic value of the baseband data is 0 is taken as sin(ωt) that is the carrier itself, and a BPSK signal in a section where the logic value of the baseband data is 1 is taken as −sin(ωt) wherein the phase of the carrier is shifted by just n. Here, a BPSK signal (PSK signal) is a signal obtained by modulating a carrier based on the logic value of the baseband data.
Thus, as for a detection method for obtaining original baseband data represented with a logic value from a digital modulated signal obtained by modulating a carrier according to data, synchronous detection has been known well. With synchronous detection, an oscillation signal having the same frequency as the carrier of a received digital modulated signal is oscillated, and the original baseband data is reproduced using the oscillation signal thereof.
That is to say, with synchronous detection of a PSK signal, usually, an oscillation signal and the received PSK signal are input to a multiplier, and output obtained by calculation with the multiplier is further subjected to filtering by a low-pass filter, thereby obtaining baseband data. Here, the multiplier is employed for utilizing a mixer function for performing addition and subtraction of the frequency of an input signal, and the low-pass filter is employed for removing the components twice the frequency of the carrier, included in the signal output from the multiplier.
For example, with BPSK method, upon sin(ωt) that is an oscillation signal, and sin(ωt) or −sin(ωt) that is the received BPSK signal being input to the multiplier, as the output thereof, (1−cos(2ωt))/2 is obtained when the logic value of the baseband data is 0, and (−1−cos(2ωt))/2 is obtained when the logic value of the baseband data is 1.
The components of cos(2ωt) are removed from (1−cos(2ωt))/2 or (−1−cos(2ωt))/2 that is the output from the multiplier using the low-pass filter, as the output from the low-pass filter, there can be obtained a signal level of 1 when the logic value of the baseband data is 0, a signal level of −1 when the logic value of the baseband data is 1.
In order to perform synchronous detection of a digital modulated signal, the oscillation signal needs to be a signal having completely the same frequency as the carrier of the received digital modulated signal. Therefore, a carrier reproducing signal obtained by carrier reproduction is frequently employed as an oscillation signal. This carrier reproducing signal is obtained by taking the received digital modulated signal as two inputs to the multiplier, i.e., the received digital modulated signal, and completely the same signal as that signal being input to the multiplier, the output from the multiplier being further input to a band-pass filter, and a ½ frequency divider. Here, the multiplier is employed for utilizing a function as a frequency 2 multiplying circuit, and the band-pass filter is employed for enhancing the components twice the frequency of the carrier.
For example, with BPSK method, upon sin(ωt) or −sin(ωt) that is the received BPSK signal being input to the multiplier and being squared, as the output thereof (1−cos(2ωt))/2 is obtained in either case. Subsequently, only the components of cos(2ωt) are extracted from the obtained 1−cos(2ωt) using the band-pass filter, and further the frequency thereof is divided into ½ using a ½ frequency divider or the like, thereby obtaining the components cos(ωt) having completely the same frequency as the carrier. Subsequently, the phase or the like of the signal of cos(ωt) thus obtained is adjusted, thereby obtaining an oscillation signal sin(ωt) employed for synchronous detection of a BPSK signal.
As described above, baseband data is generated by down-converting the received digital modulated signal from the band of the carrier to the band of baseband utilizing synchronous detection and carrier reproduction, thereby obtaining the original data.
With the above-mentioned detection method of a PSK signal, the properties of a PSK signal as an analog signal are utilized well, whereby a detection device for detecting a PSK signal using such a detection method can be effectively realized by configuring the detection device with an analog circuit. Note however, in a case wherein a detection device for detecting a PSK signal using the above-mentioned detection method is configured of a digital circuit, it has been difficult to reproduce the carrier of the PSK signal by the digital circuit.
Therefore, there have been proposed several methods for detecting the original data from a PSK signal without performing carrier reproduction (e.g., see Japanese Unexamined Patent Application Publication No. 60-198945, Japanese Unexamined Patent Application Publication No. 62-183247, Japanese Unexamined Patent Application Publication No. 2-117247, Japanese Unexamined Patent Application Publication No. 6-85860, and Japanese Unexamined Patent Application Publication No. 8-317005).
With these methods, first, the received PSK signal is sampled using a sampling clock having a sampling frequency several times the frequency of the carrier of the PSK signal. Next, according to the code transition timing of the sampling data series obtained with the sampling, the phase of the carrier in the sampling data series is detected. Subsequently, in accordance with the timing of the detected phase, detection of the original data is carried out by a majority vote of the logic values of the sampling data before and after thereof. Here, in order to perform sampling at the sampling frequency twice or more the frequency of the carrier of the PSK signal, a phase synchronization circuit capable of outputting the clock of the frequency of the integral multiplication of the frequency of the input clock, i.e., a PLL (Phase Locked Loop) circuit is employed to generate a sampling clock.
Also, with a transmission device and reception device capable of both of transmission and reception of a PSK signal, and particularly with a transmission device, a PSK signal is transmitted while synchronizing the carrier and baseband data to be transmitted, and accordingly, it is desirable for a digital circuit making up these devices to operate by being driven with a driving clock having the frequency comparable to the frequency of the carrier of a PSK signal. Accordingly, with a reception device as well, the digital circuit is driven with a driving clock having the frequency comparable to the frequency of the carrier of a PSK signal, and the reception device generates the above-mentioned sampling clock having a frequency of the integral multiplication of the frequency of the carrier using the driving clock.
In a case wherein sampling of a PSK signal is performed using the sampling clock generated from the driving clock, and further detection is performed using the sampling data obtained as a result thereof, reproduction of the carrier at the analog circuit is replaced with detection of the phase of the carrier in the sampling data series, and synchronous detection itself is replaced with detection of the original baseband data by a majority vote of sampling data.
Here, the phase of the carrier is detected based on the timing of reverse of code of the sampling data, and the original baseband data detected using the sampling data, and a phase-locked loop is operated with the information of the phase of the carrier obtained as a result thereof. Also, with detection of baseband data according to a majority vote, the first and later half cycles of the sampling clock having the same phase as the phase of the carrier, i.e., the sampling data in one cycle worth in combination thereof is employed, and detection of the original baseband data is performed based on a majority vote of the logic value of the sampling data thereof. That is to say, the logic value of the baseband data is determined from the logic value 1 or 0 in a first half cycle of the sampling data, and the logic value 1 or 0 in a later half cycle thereof.
Thus, a PSK signal is sampled at a sampling frequency several times the frequency of the carrier of the PSK signal, and detection of the original baseband data can be performed based of a majority vote of the logic value of the sampling data obtained as a result thereof without performing carrier reproduction. With such a detection method, the properties of a PSK signal as an analog signal are not employed, so the precision thereof is not high as compared with a method for employing the properties of an analog signal, but a detection device can be designed with digital circuits, and accordingly, there is no need to employ special devices necessary for analog circuits, circuit scale and power requirement can be suppressed, and stable operations can be expected.